1. Field of the Invention
The present invention relates to an actuator in which a piezoelectric bimorph allows adjustment of the mounting height of a magnetic head on a rotary drum, for example, in a video tape recorder (VTR), and to a magnetic head device including the actuator.
2. Description of the Related Art
FIG. 10 shows an example of a structure of a rotary head device (magnetic head device) used in known video tape recorders (VTRs) and digital audio tape (DAT) recorders. Referring to FIG. 10, a rotary head device 1 includes a fixed drum 2, a rotary drum 3 that is rotatably supported by the fixed drum 2 and is rotated by a motor (not shown), and a magnetic head 4 slightly protruding from a peripheral surface of the rotary drum 3.
During tape recording and reproduction in the rotary head device 1 having the above-described structure, the rotary drum 3 is rotated at high speed, and a magnetic tape 5 is wound around a part of the peripheral surface of the rotary drum 3, as shown in FIG. 11. In this case, the magnetic head 4 obliquely touches the magnetic tape 5 by so-called head tracing, as shown by the arrow in FIG. 12, and performs magnetic recording on recording tracks 5a or reproduces electrical signals recorded on the recording tracks 5a. 
FIG. 13 is a cross-sectional view of a contact portion between the magnetic head 4 and the magnetic tape 5. The magnetic head 4 is fixedly held on the rotary drum 3 with a mounting member 4a disposed therebetween. In this case, traceability of the magnetic head 4 with respect to the pattern of the recording track 5a on the magnetic tape 5 greatly varies in accordance with the mounting height of the magnetic head 4 in the y-direction.
For example, when the mounting height of the magnetic head 4 shifts, a path A of the magnetic head 4 on the magnetic tape 5 deviates from the pattern of the recording track 5a, as shown in FIG. 14. Consequently, precise tracing is difficult
Therefore, it is necessary to precisely adjust the mounting height of the magnetic head 4. On the other hand, the winding state of the magnetic tape 5 around the rotary drum 3 is optimized, for example, by means of tape guides 6 and a tape tensioning mechanism (not shown).
For that purpose, it is known to adjust the mounting height of the magnetic head 4 by means of a piezoelectric actuator 7 serving as a mounting member for mounting the magnetic head 4 on the rotary drum 3, as shown in FIG. 15. The piezoelectric actuator 7 shown in FIG. 15 utilizes an inverse piezoelectric effect that produces strain proportional to the electric field applied to piezoelectric ceramics. In the piezoelectric actuator 7, piezoelectric plates 7b and 7c, each of which is formed of a piezoelectric thin plate, are respectively bonded to both surfaces of a central member 7a. The central member 7a is formed of a thin plate that is resistant to expansion and contraction.
The central member 7a is made of a light material having a low coefficient of linear expansion, such as a titanium alloy, stainless steel, or a carbon fiber resin material in which carbon fiber and resin are mixed at a predetermined compound ratio. The piezoelectric plates 7b and 7c are made of, for example, ceramic or resin having piezoelectricity. Electrodes (not shown) are formed on surfaces of the piezoelectric plates 7b and 7c by plating, vapor deposition, baking, or other means, and are subjected to polarization. This allows the piezoelectric actuator 7 to be displaced in accordance with the direction and intensity of the applied electric field.
One end (right end in FIG. 15) of the piezoelectric actuator 7 is fixedly sandwiched between fixing members 8. One or both of the fixing members 8 are fixed to the rotary drum 3 by a screw or the like. For example, the magnetic head 4 is fixed to a lower surface of the other end (left end) of the piezoelectric actuator 7 by an instant adhesive, and is reinforced by ultraviolet curable resin, as necessary.
In the piezoelectric actuator 7 having the above-described structure, a driving voltage is applied from a power source (not shown) between the electrodes of the piezoelectric plates 7b and 7c and the central member 7a. In this case, the piezoelectric plate 7b contracts in the longitudinal direction because an electric field is applied thereto in the same direction as (or opposite to) the polarizing direction, and the piezoelectric plate 7c expands in the longitudinal direction because an electric field is applied thereto in a direction opposite to (or the same as) the polarizing direction.
Therefore, the piezoelectric actuator 7 is bent upward at the left end, and is displaced as a whole in the y-direction, as shown in FIG. 16. Since the amount of deformation due to this bending is controlled in accordance with the intensity of the electric field, an electrically controlled actuator is achieved.
The magnetic head device shown in FIGS. 10 and 16 is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-126318. FIG. 17 is a perspective view of a magnetic head device described in this publication. The magnetic head device includes a trapezoidal piezoelectric bimorph 10, a fixing member 11a that fixes the piezoelectric bimorph 10 and that has screw holes used to fix the piezoelectric bimorph 10 to a drum, fixing members 11b and 11c that sandwich the piezoelectric bimorph 10 and a thin plate 12 therebetween, the thin plate 12, a VH flexible board 13a bonded to the thin plate 12, a magnetic head 14, a head base 15 that allows the magnetic head 14 to be fixed to a leading end of the piezoelectric bimorph 10, and coils 16 provided in the magnetic head 14.
The piezoelectric bimorph 10 includes piezoelectric plates 10a and 10b, an intermediate electrode 10c, and surface electrodes 10d and 10e. 